1. Field of the Invention
The present invention relates to a silicon-on-insulator (SOI) substrate. The present invention particularly relates to bonding SOI technology and relates to a method for manufacturing an SOI substrate by bonding of a single-crystal or polycrystalline semiconductor layer to a substrate having an insulating surface such as a glass substrate. The present invention also relates to a doping apparatus for ion irradiation.
2. Description of the Related Art
With the development of VLSI technology, higher integration, higher speed, and less power consumption are strongly demanded, and in order to achieve these, SOI structures attract attention. This technology allows an active region (channel formation region) of a field-effect transistor (FET), which has been formed of bulk single-crystal silicon, to be formed of a single-crystal silicon thin film. It is known that a MOS field-effect transistor manufactured using an SOI structure has lower parasitic capacitance than that manufactured using a conventional bulk single-crystal silicon substrate, which is an advantage in increasing speed.
Examples of SOI substrates include SIMOX substrates and bonded substrates. For example, an SOI structure of a SIMOX substrate is obtained by implantation of oxygen ions into a single-crystal silicon substrate and by heat treatment performed at 1300° C. or higher to form a buried oxide (BOX) layer, whereby a single-crystal silicon layer is formed on the surface. For a SIMOX substrate, oxygen ion implantation can be controlled precisely and thus a single-crystal silicon thin film having an even thickness can be formed with high control; however, there are problems with time and costs because a long period of time is needed for oxygen ion implantation. In addition, there is another problem in that a single-crystal silicon layer is likely to be damaged during oxygen ion implantation.
An SOI structure of bonded substrates is obtained by bonding of two single-crystal silicon substrates (a base substrate and a bond substrate) to each other with an insulating layer interposed therebetween and by thinning of one of the two single-crystal silicon substrates (the bond substrate), whereby a single-crystal silicon layer is formed. As a thinning method, a hydrogen ion implantation separation method is known. A hydrogen ion implantation separation method is a method in which hydrogen ions are implanted into a silicon substrate to form a microbubble layer at a predetermined depth from the substrate surface, and using the microbubble layer as a cleavage surface, a thin single-crystal silicon layer can be bonded to another substrate (see Reference 1: Japanese Published Patent Application No. 2000-124092).
On the other hand, there is an attempt to form a single-crystal silicon layer over a substrate having an insulating surface such as a glass substrate. For example, one known example of SOI substrates in which a single-crystal silicon layer is formed over a glass substrate is disclosed by the present applicant (see Reference 2: Japanese Published Patent Application No. H11-163363).